The discussion here is of the low speed benefits of AC traction motors. There is also a high speed benefit, better speed control. DC motors and AC induction motors change speed with load. AC synchronous motors maintain the same speed from zero to full rated load.
This speed is determined by the AC frequency. Since the DC to AC chopper frequency is probably computer controlled, the potential exists for rather fine speed control.
In addition to the TGVs, don't forget the other big application of synchronous traction motors built by Alstom. SNCF's BB26000 "Sybic" (Synchronous bi-courant) locomotives. The other interesting point about early Alstom AC traction drives is that they used Current Source Inverters, while German and Swiss builders used Voltage Source Inverters.
in the spirit, without the wit, of SF Chron's Herb Caen...don"t know destiny of the LIRR AC FL9s...but was it Brown-Bovari....U25B's on SP had a WS control that shot 10psi into the ind br to correct the WS(pin)....seemed to work with slow unit unloading...Wheel Creep worked for the SP GP60's.....down to 13 mph on Cuesta out of San Luis Obispo....screeching, put in your earplugs tighter-screeching, loudest, painful...close...from no other engine.....two .GP40s and a 24 car ballast train getting onto the main track was way too long, this in my arrogant opinion....on the Altamont at lowering speed with blinking evry 8 seconds or so WS light....Whaa?.....pulled the MU cable, the pilot engr ran the trailing eng, highly skillfully up to its WS amperage and backed off the throttle.
My 1990's Boise improved rebuild GP40 pegged, almost, at 1500 amps...2mph or less.....ACE left Stockton, we were three miles from getting in the clear at Midway,a CTC siding,..... and 40 minutes ahead of the ACE....
He caught our yellows and a red,before we cleared in Midway....I thought the Dash-3 WS control wouldn't get me over the siding switch's frog.... it did....a mile at mimutes but making it wth what was unbelievable adhesion.....it was a 38 car train...2 healthy GP40s probably would have had little trouble on the west slope of the former Western Pacific Altamont.... my major exception to the WS(lip) problem is if Dynamic Brake, because of sliding wheels, allows speed to increase to an unsustainable down-grade acceleration....Does it matter...DC or AC.....lose DB at the speed at which the train brakes can't arrest acceleration powered by gravity
The LIRR had two (I believe) FL-9's, ex-Metro-North, ex-Conrail, ex-PC, ex-NYNH&H, rebuilt with AC traction motors (asynchronous). I think Brown Brevori may have been involved. Anyone know what happened to them? Still around? Used at all? Scrapped? Museum? Adarondak Scenic?
Jay: Thanks. The key is "substantial loss", of course; you've got to slow the slipping wheel down and get it back on the stable side of the creep/adhesion curve. With microprocessor controls and AC motors it's a lot easier to accomplish. Some reduction is inevitable, just because of the pure mechanics and inertias involved, but I certainly agree that the modern systems do an amazingly good job.
Dave Phelps
Dave, I agree with everything that you said except for the necessity of dropping an axle's tractive effort nearly to zero whenever the axle is on the negative side of the creep curve. I believe that some adhesion control systems are capable of reducing creep back to the set point without a substantial loss of tractive effort.
Jay (and Erik):
Historically, the inherent charateristic of series DC traction motors made it close to impossible to utilize wheel creep (sometimes called "microslip") because stabilzing the control was extremely difficult. When a DC series motored axle begins to slip, it is unloaded and tries to run away. With AC induction motors the beginning of mechanical slip reduces the electrical slip, which in turn drops the tractive effort, and it tends toward self-correction. Thus, the kind of adjustment you describe is much easier to implement because the electromechanical system is helping you instead of fighting you.
There still remains the terminology/nomenclature issue. Although it's not universal, the tendency has been to refer to the controlled maintenance of slight wheel creep, which works on the positive, stable side of the creep/adhesion curve, as exactly that, "wheel creep." A loss of adhesion sufficient to go over the top to the unstable, "run away" side of the creep/adhesion curve, is called a "wheel slip", and the only way to correct it is the historical method of dropping the tractive effort to zero (or nearly so) very quickly and waiting for the wheel to regain rolling adhesion before re-applying tractive effort, or increasing available adhesion with sand, or both.
I'm having difficulty relating the "onset of wheelslip" concept to the concept of wheel creep. Adhesion-limited tractive effort is maximized by regulating wheel creep so that it remains as close to its optimal rate as possible as rail conditions change. In other words, when wheel torque is reduced as a result of worsening rail conditions, the traction control system will attempt to restore torque by adjusting the creep rate either upward or downward. So wheelslip itself (i.e. creep) won't necessarily cause a reduction in torque. It will only do so when the rate of slippage is either decreasing (or remaining constant) when it should be increasing or increasing (or remaining constant) when it should be decreasing.
Speaking of electrical versus mechanical slip...
One significant advantage of AC induction motor over DC series motors is that the torque of the AC motor drop far more rapidly at the onset of wheelslip than is the case for a DC series motor. This greatly simplifies the problem of trying to get the last bit of adhesion limited tractive effort.
- Erik
Johnny: The Republic article is a good overview. However, to repeat my caution from earlier, they don't do a good job of differentiating between the electrical slip in the induction asynchronous traction motors and the mechanical slip between the wheel tread and the rail. If you read VERY carefully, being on the lookout for the issue, you'll recognize it and keep them straight.
They are also a little cavalier about keeping straight tractive effort limited by available adhesion and tractive effort limited by the action of the control system. Again, if you read VERY carefully you should be ok.
It's not appropriate to do a detailed nit-picking of a generally "ok" article, and I'm not going to, but do be aware that there are simplifications and generalizations in it.
Sam, I'm with you--but I can drive a nail straight; do you want me to help you the next time you want to nail something together?
I had read the various discussions of AC power versus DC power in Trains over the years, but did not comprehend the advantages of AC power until I read the Republic article.
Johnny
Thanks,, Sam. And I think we learn something about transportation economics, both freight and passenger, from you.
Even though I still believe in more emphsis on national priorities vs. economics than you do, I do indeed recognize the honesty with which you approach problems.
I am not an engineer. Shoot, I can barely drive a nail straight. But the discussions in this thread have helped me understand the difference between AC and DC power. It has been a great discussion for me. I have learned a lot.
On several occasions I have asked for the advantages of AC vs. DC. Now I have finally gotten some answers that I understand. This is one of the advantages of being able to participate in these forums.
Oh my. Dave, with all due respect, you're mixing up induction motors with hysteresis motors. Everything you say is correct for induction motors. Please check your reference sources.
Perhaps it's time to give our poor readers a respite from this. If you want to contact me off-line please feel free, drperiepa@aol.com.
Without doing the research and relying on my memory, the torque is NOT modest when they run esasynchronously and that is the way they are used in locomotives,, rail cars, and electric drive buses. In freight locomotives, they run asynchrnously just about all the time.
Intersting history:
DC commutator motor with field in series with commutator, and similar low-frequecny AC motor that can also run on DC: Largely self-corrcting with changes in load, power control requires loss of power in resistors when drawing power from thrid rail or overhead wire or battery, or control of prime mover (tranistions and field shunting mitigate the problem in both cases, but by no means elminates the resistive losses in the third rail and overhead wire and battery applicatoins.)
Chpper control then introduced to eliminate resistor grid inefficiency.
Then ac generators replacing dc in diesel-electrics to remove brush and commutator wear, allow simpler power control, but keeping dc motors.
Then AC hysterises motors with variable speed inverters and sophisticated computer control
Now synchrnous motos with even more sophisticated and faster-acting computer control.
The trend is less self-correction in the motor and more reliance on control sophistication.
Dave:
More research needed regarding hysteresis motors. Try http://www.groschopp.com/synchronous-vs-asynchronous/ and http://encyclopedia2.thefreedictionary.com/Hysteresis+Motor. Synchronous. No dropped "a". However, do see https://en.wikipedia.org/wiki/AC_motor#Hysteresis_synchronous_motor, which discusses that although inherently synchronous, the relatively modest torque capability of hysteresis motors means that they can and do also run asychronously.
I stand corrected. Permanent-magnet motors do not have slanted bars, neither the pole structure nor the magnets. The magnet structure itself is more forgiving then the induced current in a squiral-cage induction motor, and the control system to accomodate varying loads and speeds can be less critical,. This is true for both inside-out wheel-motors and those using slip-rings with rotating coil armatures.
Apparently, Alstom's technology has outstript my imagination. If they are your friends, they certainly can be proud of the light rail cars for Jerusalem. I would like the maintencance to be a bit better though, since on occasion I can hear gear whine spoiling a truly quiet performance. Not usual, but just occasoinally.
But you do note that an hyserisis motor is asynchronous and not synchronous. Easy for a copyist to drop the a.
Dave, let me simply say that you might want to do some research on this. My friends at Alstom would be very puzzled by your statement that they do not build/apply synchronous motors (for railway traction).
Synchronous motors, initially wound rotor with slip rings and now permanent magnet, are used extensively on TGV-Atlantique (and its Spanish derivative) and Thalys.
Yes, the control system has to be sophisticated, and it is. For traditional industrial applications, your caution about the reaction of a synchronous motor to load changes is correct, but modern-day railway traction applications are a far cry from "your grandfater's a.c. drive." However, the TGV applications have been the only extensive synchronous motor applications. The rest of the wold has preferred to stay with ascynchrous motor applications. PM motors are coming on strong, though, based on the most recent technical papers, especially in automotive applications.
With Kalmbach now taking orders for "Locomotive 2013", it's interesting to speculate on what technology will be featured in "Locomotive 2023", ten years from now!
For traction purposes, Alstom does not build synchronous motors, and they have slanted bars. Or the pole magent structure can be slanted instead. (Even the permanent magnet variety.) Calling any of the motors with slanted bars or pole pieces synchronous is someone's copy error, and this can be an error by a publcist for the manufacturer. Even slippage of 1% rotational speed (quite normal for normal load at top speed) qualifies the motor as non-synchronous. Generally, purely synchronous motors are used for fixed loads, as in fans, but are a poor choice for transportation, highway or rail, because of the need to accomodate sudden and unexpect load variations. These can cause a synchronous motor to stall while the slanted bar (or pole structure) motor will simply increase slip (decrease speed) and increase current at the same time.
Incidentally, I suspect that Alstom is actually using motors built by the German firm Magnet-Motor. They and Stored Energy Systems, Derby, England, have pioneered permanent magnet motors. Generally, the permanent magnets are in the rotor and the rotating field is in the fixed coils. Sort of inside-out from your model railroad dc motors. They were first pioneered on low-floor buses as "wheel motors" and then on low-floor light rail cars, and railway shop battery shunters.
I think Google is in error to call a hysteresus motor a synchronous motor. But apparenly since my involvement in this kind of engineering, the term Induction motor has been extended to include both the motors that prduce torque with slip and those that lock in at rotation speed with maximum torque near 90-degree phase lag.
I could conceve of a control system and inverter system so sophisticated that sychronous motors could be used for rail and highway applications, but it would be a far more sophisticated control and inverter system than those I believe are actually in use. In would have to respond in microseconds to changes in load and have sophisticated prediction allegorims to insure that the rotating magnetic field produced by the fixed coils meets the demands both in terms of rotational speed and current. Slanting the rotating bars or the pole pieces makes the job a lot easier and allows torque with slippage.
The motors used on most modern locomotives are 3-phase Asynchronous design, though Alstom is trialling a 3-phase Synchronous permanent magnet motor in their AGV trainsets that they are producing for NTV in Italy.
Following up, I did some Google searches. I confirmed that my memory is right, hysterisis motors are synchronous motors. I also found...ahem...lots of images of induction motors with skewed rotor bars.
The explanations of various types of a.c. motors on Wikipedia are quite good, and generally much more succinct than I tend to be.
I'd need to know specifically what motors you're referring to; none of the other posters mentioned specific motors or applications. I've never seen an induction traction motor with skewed rotor bars, and while "never say never" I need more information. There have been synchronous traction motors used; some of the French TGVs have them. More recently, they've gone from tradtional wound-rotor synchronous traction motors to permanent magenet motors. Are some of those motors what you have in mind? A hysteris motor is synchronous, if I'm recalling correctly.
Actually, no, an induction machine does NOT turn at the same speed as the rotating magnetic field from the stator when it's under motoring or braking load. At synchronous speed, an induction machine produces zero torque. A synchronous machine does, because its rotor has an externally-applied dc current producing a magnetic field that interacts with the rotating field; once the motor is up to speed and "locks in", the interaction produces torque. The electrical angle between the two increases as a function of load until 90 deg. is reached, which is the maximum torque point. (I suspect that's what you meant by "lag".)
An induction motor is a case in point of an asycnchronous motor. The rotor magnetic field that interacts with the stator magnetic field comes from currents that are "induced" by transformer action by the fact that the rotor bars, rotating at a slightly different speed than the stator magnetic field, cut the lines of flux and thereby have current flow induced. This difference in speed is the electrical slip, and if it's negative we have a motor, while if it's positive we have a generator, which in railway applications gives us dynamic braking.
We may have a nomelature hiccup...
DAVE PHELPS, A BEAUTIFUL ANAND THOROUGH EXPLANATION
EXCEPT ONE MINOR POINT:
Strictly speaking these are not induction motors. The rotating bars in induction motors go straight across, and these motors have slanted bars. Why?
Because an induction motor turns at the same speed as the rotating magnetic field produced by the specifici frequency and phase differences applied to the fixed coils. When load is applied, the rotating bars will lag more and more until torque limit is reached and then stop carrying the load.
I believe the motos are called hysterises non-synchronous motors, typified by slanting bars, and using the residual magnetism in the magnetic cores of the fixed coils to continue to profice torque as the rotation of the bars slips more and more under load.
But perhaps the term induction motor is now applied to both synchonous and non-synchronous motors. Live and learn.
You've gotten some good answers, especially the one from Mr. (?) beaulieu. However, nobody has directly answered your question, so let me give it a try - a "verbal block diagram" with comments.
- It all starts with the diesel engine, which produces mechanical power, torque at a speed. Since its torque ability is proportional to its speed, to get up to rated gross engine output, notch by notch, it's run at higher and higher speed (usually, unless there's a HEP generator attached. Then it has to run at the proper speed for the HEP output [usually 900 rpm] and torque is limited.)
- Next, the engine drives the main generator, always these days an a.c. generator/"alternator" for reasons others have explained. Since the alternator is spining at the same speed as the engine, its output frequency is dependent on the number of electrical poles in the alternator and the engine speed.
- Since the point of the electrical transmission is to have full horespower available over a wide speed range, we must do something to allow the a.c. traction motors to create that power as tractive effort at the rail over that same wide speed range. For reasons Mr. Beaulieu expained and the following paragraph from the Republic link supplements, "The AC system, however, operates in a very different fashion. The variable frequency drive creates a rotating magnetic field which spins about 1% faster than the motor is turning. Since the rotor cannot exceed the field speed, any wheel slip is minimal (less than 1%) and is quickly detected by the drive which instantly reduces load to the axle." the fixed (for any given notch) frequency from the traction alternator cannot be applied directly to the traction motors.
- The way to apply voltage and frequency to the a.c. traction motors that's appropriate is by a variable voltage, variable frequency (VVVF) inverter. (Applying the voltage causes current to flow; technically it's the current that produces the torque, of course.)
- I deliberately skipped one "verbal block", to make a point. The feed to the a.c. traction motors is from an "inverter", DC to AC inversion. So, to get the DC that the inverter likes, we rectify the a.c. from the alternator to d.c.; this resulting d.c. output is referred to as the "d.c. link".
- So, we control the voltage and frequency applied to the a.c. traction motors so that the "electrical slip" mentioned in the Republic paragraph is created. (DO NOT CONFUSE THIS WITH MECHANICAL WHEEL SLIP. Unfortunately other Republic paragraphs fall into that trap, at least somewhat.). It is this electrical slip that induces current flow in the rotor (now you know why they're called induction motors!); the interaction of the magnetic field in the rotor with the one in the stator that the inverter's current produced creates the torque that through the gearing and the wheel diameter makes tractive effor and off we go!
Well, you asked an engineer what time it was, and I told you how to make a watch. If I've told you more than you wanted to know, my apologies, but I felt that your very pertinent question deserved a complete answer. Thanks for asking it!
Lab Since DC is used between the AC generator and the AC traction motors, a DC generator could drive AC traction motors using the same DC to AC conversion.
Since DC is used between the AC generator and the AC traction motors, a DC generator could drive AC traction motors using the same DC to AC conversion.
That would be theoretically possible but not practical. Main generators (DC) gave way to main alternators (AC) in the mid to late 1960's when horsepower per unit reached the 3000 mark. Size constraints were one factor while wiring and other electrical factors also came into play.
Palooka, I think this seems like a pretty good description:
http://www.republiclocomotive.com/ac_traction_vs_dc_traction.html
_____________
"A stranger's just a friend you ain't met yet." --- Dave Gardner
Well put.
The reason for interposing a DC Link is that the Traction Motor rotational speed is determined by the frequency of the AC input irrespective of the input current. On the other hand the output frequency of the Traction Alternator is directly proportional to the speed of the diesel engine. Because of this high power output (high diesel engine speed) would only be possible at high movement speeds. The interposing of the DC link and the four quadrant traction inverters allows the frequency of the current supplied to the traction motors to be controlled independently of the diesel engine speed. In addition because they are 4-quadrant inverters when setup for dynamic braking they can convert the energy returning from the traction motors operating in generator mode into DC for the braking resistors.
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